Intracellular replication of Legionella pneumophila is culminated in lysis of the host cell and bacterial egress, associated with extensive necrosis in the alveolar spaces. Our data show that expression of the pore-forming toxin/cytolysin by intracellular L. pneumophila is triggered upon termination of intracellular replication, and mutants defective in the pore-forming toxin replicate intracellularly similar to the parental strain but remain "trapped" within and fail to egress from the host cells. We have identified the pore-forming toxin and a co-factor required for its export, both encoded by a bi-cistronic operon. Our hypothesis is: One of the proteins is the structural toxin constituting the "egress pore" while the other is a cofactor for export of the cytolysin as well as other virulence effectors. The cytolysin of L. pneumophila plays a major role in pulmonary pathology, and has the potential of an effective vaccine against Legionnaires" disease. To test this hypothesis, our Specific Aims are to study the following: 1) Identification of functional domains in both proteins and identification cytolysin-interacting proteins required for bacterial egress; 2) Effects of insertion of the cytolysin pore on cell biology, including the phagosome and cytoplasmic organelles; and 3) Role of the cytolysin in acute inflammation, and its potential use as a vaccine. Significance of the proposed work: Our proposed studies will uncover at the molecular, biochemical, and cellular levels of a novel mechanism by which L. pneumophila becomes cytotoxic, kills and egresses from the spent host cell after its exploitation for proliferation. Our studies will evaluate the potential use of the toxin as a vaccine against Legionnaires' disease. The pore-forming toxin of L. pneumophila may become a tool for various biological studies (similar to many other bacterial toxins) and also for therapeutic purposes such as its potential targeting to malignant cells. Our studies on L. pneumophila may provide a paradigm for other vacuolar intracellular pathogens such as Mycobacterium, Salmonella, and Chlamydia.